Global optical/infrared���X-ray correlations in X-ray binaries: quantifying disc and jet contributions

Abstract

The optical/near‐infrared (OIR) region of the spectra of low‐mass X‐ray binaries (XBs) appears to lie at the intersection of a variety of different emission processes. In this paper we present quasi‐simultaneous OIR–X‐ray observations of 33 XBs in an attempt to estimate the contributions of various emission processes in these sources, as a function of X‐ray state and luminosity. A global correlation is found between OIR and X‐ray luminosity for low‐mass black hole candidate XBs (BHXBs) in the hard X‐ray state, of the form LOIR∝L0.6X. This correlation holds over eight orders of magnitude in LX and includes data from BHXBs in quiescence and at large distances [Large Magellanic Cloud (LMC) and M31]. A similar correlation is found in low‐mass neutron star XBs (NSXBs) in the hard state. For BHXBs in the soft state, all the near‐infrared (NIR) and some of the optical emissions are suppressed below the correlation, a behaviour indicative of the jet switching off/on in transition to/from the soft state. We compare these relations to theoretical models of a number of emission processes. We find that X‐ray reprocessing in the disc and emission from the jets both predict a slope close to 0.6 for BHXBs, and both contribute to the OIR in BHXBs in the hard state, the jets producing ∼90 per cent of the NIR emission at high luminosities. X‐ray reprocessing dominates the OIR in NSXBs in the hard state, with possible contributions from the jets (only at high luminosity) and the viscously heated disc. We also show that the optically thick jet spectrum of BHXBs extends to near the K band. OIR spectral energy distributions of 15 BHXBs help us to confirm these interpretations. We present a prediction of the LOIR–LX behaviour of a BHXB outburst that enters the soft state, where the peak LOIR in the hard state rise is greater than in the hard state decline (the well‐known hysteretical behaviour). In addition, it is possible to estimate the X‐ray, OIR and radio luminosity and the mass accretion rate in the hard state quasi‐simultaneously, from observations of just one of these wavebands, since they are all linked through correlations. Finally, we have discovered that the nature of the compact object, the mass of the companion and the distance/reddening can be constrained by quasi‐simultaneous OIR and X‐ray luminosities.

The Reflares and Outburst Evolution in the Accreting Millisecond Pulsar Sax J1808.4–3658: A Disk Truncated Near Co-Rotation?

“…The non-detections are not reported. 11 The correlation between optical and X-ray flux found by Russell et al (2006Russell et al ( , 2007 are empirical rather than theoretical.…”

“…In Figure 4 we plot the de-reddened optical to X-ray (unabsorbed) luminosity relation for the Swift observations of the 2005-2008 outbursts plus additional data referring to other neutron star LMXBs where these two quantities are measured (and found to be correlated, see Russell et al 2006Russell et al , 2007. 11 We used the 2-10 keV energy band rather than the 0.3-10 keV in order to match the analysis of Russell et al (2006) done for the other neutron star LMXBs used in Figure 4.…”

“…11 We used the 2-10 keV energy band rather than the 0.3-10 keV in order to match the analysis of Russell et al (2006) done for the other neutron star LMXBs used in Figure 4. Some of the 2008 data lie slightly above the correlation with a slope that is somewhat flatter than recorded for other neutron star LMXBs.…”

“…Russell et al (2006Russell et al ( , 2007 estimated the coefficient β for neutron stars (assuming a radiatively efficient accretion flow, L M…”

Disc reflection and a possible disc wind during a soft X-ray state in the neutron star low-mass X-ray binary 1RXS J180408.9–342058

“…Such a low inclination could account for the lack of orbital signatures in the optical data (Baglio et al 2016), as ellipsoidal modulations would be small. Secondly, the orbital period of LMXBs correlates with their absolute visual magnitude and X-ray luminosity (van Paradijs & McClintock (1994), see also Russell et al (2006)). We measured an X-ray luminosity of L0.7−35≃2 × 10 37 erg s −1 on 2015 April 14, whereas Baglio et al (2016) reported an apparent visual magnitude of ≃17 mag on 2015 April 24 (see their table 2).…”